Additional information:

Salmo trutta

What do they look like?

Adult brown trout are between 18 to 56 cm long, and can weigh from 9 to 24 kg. Brown trout have a large fin on their back, with a much smaller fin on the back near the tail. On their underside, they have 3 sets of fins. There are two fins behind the head, a fin on the middle of their belly, and a fin close to the bottom of the tail. Brown trout have a large mouth, and males that are ready to mate develop a hook-like bump on the lower jaw called a kype. The top of the trout is an olive color with brown and black spots, while the belly is tan or yellow. The sides of the fish have many orange and red spots ringed with a light blue. Young trout have less spots than the adults, but they do have "parr marks", which are large dark circles along the side of the fish. These marks disappear as they get older. Male brown trout have a larger head than the females, while the females have a larger abdomen. (Jenkins and Burkhead, 1993; Klemetsen, et al., 2003; Trautman, 1981)

What kind of habitat do they need?

Most brown trout live in deep, slow-moving streams, as well as lakes. Recently hatched fish are found at the bottom of the river or lake, while juvenile brown trout are in shallow water, and adults are usually in the deeper water. They prefer to have rocky, gravel-covered bottoms of the rivers and lakes that they live in. Some brown trout are anadromous, meaning that they live part of their life in freshwater, and another part of their lives in saltwater oceans. Anadromous adult trout live in the ocean, and return to freshwater to lay eggs. The young trout live in the freshwater lakes and streams until they become adults. Other brown trout spend their whole lives in freshwater lakes and streams. (Cunjak and Power, 1986; Heggenes, 1996; Klemetsen, et al., 2003; Shirvell and Dungey, 1983; Stauffer, Jr., et al., 1995)

How do they grow?

Brown trout eggs are buried in a gravely nest called a redd, and stay there for 1 to several months before hatching the next spring. When the egg hatches, the first stage emerges, called an alevin. An alevin has a yolk sac attached to its stomach, which is used as a food source. Alevins stay in the gravel nest until they use up all their yolk. They then become the second life stage, called a fry. Fry leave the nest, but still stay close as they look for food. The next stage is called a parr. Parr go to other areas of the river to look for food, and also establish their own territories. They then become adults. Most brown trout will stay in their freshwater river or lake when they become adults. Some brown trout will swim out to oceans when they become adults. This transition to saltwater from freshwater is called smoltification. The skin of the trout becomes silvery in color when going to sea. (Jenkins and Burkhead, 1993; Klemetsen, et al., 2003)

How do they reproduce?

Males will often fight each other over a female mate. Two males will swim side by side and shake their bodies, to try and push the other male away. Males will also charge each other with an open mouth, sometimes crashing into each other. Males also sometimes bite each other. When one male wins the female, he will chase off the losing male. Females also play a part in choosing mates. They likely select their male mate based on his fin size. The larger the fin, the more likely the female is to choose to mate with him. (Petersson, et al., 1999)

All brown trout return to the stream or river that they were born in to mate. Trout that stay in freshwater their whole lives are ready to mate anywhere from 1 to 10 years of age. Trout that go to the ocean when they are adults become able to mate while at sea, and they return the next season or in 2 or 3 years to the stream of their birth to mate. Mating takes place from autumn to winter. Females choose a spot in the gravel or stones at the bottom of the stream to dig a nest and lay their eggs in. The female lays between 300 to 1500 eggs in several nests. Fertilization of the eggs takes place outside of the female body, so once the eggs are laid, the male swims above the nest and releases his sperm to fertilize the eggs. The female then buries the eggs with stones or gravel. (Jonsson, 1989; Klemetsen, et al., 2003)

Females provide nutrients in the yolk in their eggs for the young fish to grow and develop on. Females also protect their eggs by covering them with stones. However, after the female covers her eggs, she leaves and does not give any more parental care. Males also do not provide any care, though if the eggs are in their territory, they will defend their territory, and unintentionally protect the eggs. (Klemetsen, et al., 2003)

Parental Investment

pre-hatching/birth

provisioning

female

protecting

female

How long do they live?

The oldest known brown trout lived to be 38 years old. More often, brown trout live anywhere from 3 to 20 years. Brown trout that swim to the ocean when they reach adulthood do not usually live past 10 years of age. (Jonsson, et al., 1991; Klemetsen, et al., 2003)

How do they behave?

Brown trout migrate back to the streams and rivers that they were born in to mate. Some trout will also migrate to oceans when they reach adulthood, while others stay in their freshwater streams. They are active during the day, and also at dawn and dusk. They are also social animals, and males defend their territory all year. During mating season, when males fight over females, they establish that some males are dominant over other males. Those males that win mates are more dominant than the losing male. By participating in many fights, males establish a ranking of dominant males. (Klemetsen, et al., 2003)

Home Range

Brown trout have their own territories that they defend. Their territories do not have a set size, but instead depend on how much space each individual is able to defend from other trout. Larger trout can defend more space than smaller trout, and have larger territories. (Elliott, 1990; Petersson, et al., 1999)

How do they communicate with each other?

One of the main ways brown trout communicate is with pheromones, which are chemicals they produce. When females are ready to mate, they release pheromones that males can detect. In response to these pheromones, males will become more aggressive and play-fight with other males. Like all fish, brown trout can sense changes in their environment by their lateral line system. The lateral line systems is made up of sense organs that can detect vibrations, changes in the water current, and pressure differences. They can use the information collected by their lateral line system to find food or detect predators.

What eats them and how do they avoid being eaten?

Brown trout are preyed upon by birds, mammals and other fish. Common mammalian predators are humans, river otters (Lutra sp.), and mink (Neovison sp. or Mustela). Common bird predators include mergansers (Lophodytes sp.), the great blue heron (Ardea herodias), pelicans (Pelecanus sp.), and kingfishers (Alcedines). Brown trout eggs are also easy for predators to eat, since they are left unguarded in nests. Brown trout that live in the ocean can be eaten by seals and large fish such as cod (Gadus sp.).

What roles do they have in the ecosystem?

Brown trout are an introduced species in much of the world, meaning that they live in areas that are not originally from and were brought there by humans. Because they are not native to many areas, they can cause problems for native fish and other organisms. Brown trout use resources that native animals would have used otherwise, making it more difficult for the native animals to survive. Brown trout are responsible for the decrease in the population size of other fish, invertebrates, and amphibians.

Brown trout can be infected by many parasites, including flatworms such as Diplostomum sp., Posthodiplostomum cuticola, Sphaerostomum globiporum, Eubothrium crassum, and Triaenophorus nodulosus, as well as thorny-headed worms such as Acanthocephalus lucii. They are also hosts for parasitic marine crustaceans including Ergasilus sieboldi, and leeches such as Piscicola geometra.

Do they cause problems?

Since brown trout are not native to many of the places that they live, they can negatively effect ecosystems and populations of other aquatic organisms by using resources. This can cause problems for the other organisms and for the habitat as a whole, which can cause problems for humans that may depend on that habitat. ("Brown Trout (Salmo trutta): A Technical Conservation Assessment", 2007)

Are they endangered?

Brown trout are not an endangered species. Their populations are controlled by people throughout the world, including the United States. Since they live longer and are larger than many other fish species in rivers, there are some states with restrictions and rules on the size of brown trout populations, to keep ecosystems healthy and balanced. ("Brown trout management plan", 2001)